More than 95% of diesel engines will be equipped with a device for treating nitrogen oxides in the exhaust line. This will apply in the very near future to gasoline fuel engines.
In order to do this, in motor vehicles, in particular with a diesel engine, it is known to equip an internal combustion engine exhaust line with a selective catalytic reduction system having injection of reducing agent into the line, the monitoring-control unit receiving the estimates or measurements of amounts of nitrogen oxides exiting through the exhaust line at least downstream of the selective catalytic reduction system.
For the removal of nitrogen oxides or NOR, a selective catalytic reduction (SCR) system is therefore frequently used. Hereinafter in the present application, the selective catalytic reduction system could also be referred to by its abbreviation SCR, likewise the nitrogen oxides could be referred to under their abbreviation NOR, ammonia under its chemical formula NH3 and carbon dioxide under its chemical formula CO2.
In an SCR system, use is made of a liquid reducing agent intended to be introduced in predefined amounts and by consecutive injections into an exhaust line of a motor vehicle. The addition of this depolluting reducing agent treats the NOx present in the exhaust line of the heat engine of the motor vehicle. This SCR reducing agent is frequently ammonia or an ammonia precursor, for example urea or a urea derivative, in particular a mixture known under the brand Adblue®.
An SCR system typically has a tank containing an amount of liquid reducing agent, a pump for supplying liquid reducing agent to an exhaust line of a motor vehicle using an injector that opens into the exhaust line. The liquid reducing agent decomposes to give gaseous ammonia, of chemical formula NH3. The NH3 is stored in an SCR catalyst in order to reduce the NOx that are in the gases discharged by the exhaust line. This applies both for diesel vehicles and for gasoline vehicles.
Such an SCR system may be doubled or combined with one or more active or passive NOx traps. Typically, such traps store the NOx at colder exhaust temperatures. Next, the NOx are reduced, during a purging operation, under conditions of richness and heat in the presence of hydrocarbons in the exhaust. For higher temperatures, a continuous injection of fuel into the exhaust line at high frequency and under high pressure has proved more efficient than the typical alternating storage and purging operations.
An SCR system, more particularly when the reducing agent is a urea derivative such as AdBlue®, is effective between medium and high temperatures and may convert the NOx continuously. An optimized control is also required for increasing the NOx treatment efficiency and optimizing the consumptions of fuel and of reducing agent, given that these parameters are all dependent, nonlinearly, on the conditions prevailing in the exhaust and during the catalysis.
The control of an SCR system may be divided into two parts: a nominal control and an adaptive control. The nominal control sets the amount of reducing agent to be injected which is calibrated as a function of the SCR system and of the test vehicle used during the development. The adaptive control sets a multiplying correction factor for the amount of reducing agent to be injected based for the vehicle on which the SCR system is actually associated, in order to adapt the system in series with deviations and dispersions that may originate from the reducing agent injector, from the NOx sensors, from the quality of reducing agent, from the metering system, from the catalysis system or from the exhaust flow rate, etc.
It should also be taken into account that the system may have an influence on the reduction process by giving rise to more emissions of NOx or of NH3, the NH3 corresponding to the reducing agent converted but not used for the catalysis at the outlet of the exhaust line. Generally, the adaptive control uses an NH3 sensor and/or NOx sensor or works with an estimate at the outlet of an SCR-impregnated particulate filter or of an SCR catalyst, this without taking into account the case where an auxiliary SCR system is present or if there is present a catalyst for oxidation of the excess NH3 not used for the monitoring of the catalysis at the end of the exhaust line in order to avoid releasing NH3 into the environment outside of the motor vehicle.
A control of an SCR system according to the prior art enables an adaptation of a predetermined NOx treatment efficiency according to a volume ratio or a weight concentration or a level of NOx in the exhaust line, for example a mass flow rate in grams/second. If the latter is not expressly formulated in the prior art, it is however obvious to modify a control of NOx efficiency into control of the level of NOx in the exhaust line.
With reference to FIG. 1, this figure shows a logic diagram for control of an SCR system, steps 1 to 7 of which are known from the prior art. To carry out the steps, it is known to control an SCR system, which may be done by a monitoring-control unit specifically associated with this SCR system.
The monitoring-control unit operates by setting a setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line, referenced 1 in FIG. 1. The monitoring-control unit of the SCR system also calculates an amount of reducing agent 2 to be injected into the exhaust line depending on the parameters of the exhaust line and/or on the combustion parameters in the internal combustion engine, which is symbolized by the reference 3 in FIG. 1. This calculation may be carried out in an open loop, which is symbolized by the reference 5 in FIG. 1.
These parameters, in step 3, which may be taken individually or in combination, may be an amount of nitrogen oxides measured or estimated upstream of the injection of reducing agent, advantageously measured by a nitrogen oxide sensor upstream of the injection of reducing agent, a temperature in the exhaust line, a gas flow rate in the exhaust line. In addition, the amount of reducing agent injected may be adjusted by a catalysis model, which is referenced 4 in FIG. 1.
The monitoring-control unit also receives the estimates or measurements of amounts of nitrogen oxides exiting via the exhaust line at least downstream of the selective catalytic reduction system, which is referenced 6 in FIG. 1. This may be carried out by a nitrogen oxide sensor positioned downstream of the catalyst of the SCR system.
The comparison between the setpoint of the amount of nitrogen oxides per second and the estimate or measurement of the amount of nitrogen oxides exiting the exhaust line given per second enables the calculation of a deviation 7 used for correcting the amount of reducing agent 2. This may be given for example in grams per second.
For legislative standards to come in the near future, measurements of emissions under real driving conditions will be introduced. In particular, for diesel vehicles, the emissions measured by a portable emissions measurement system (PEMS) will be possible for a vehicle following random driving cycles under certain conditions making them representative of normal driving conditions.
However, in comparison with the old legislative standards for which only tests on a running bed under standard ambient conditions at a minimum weight and following a predefined speed profile were required, the new standards introduce many more operating conditions in terms of speed, involving speed levels and speed variations, varied ambient conditions, for example cold, hot or at altitude, varying dynamic parameters, such as the wind, slope of the road, air conditioning, etc., as well as vehicle weight variations varying from a minimum weight to a weight that reaches 90% of the maximum weight required by the manufacturer, etc.
This cannot be monitored with a monitoring-control unit according to the prior art described above.
For example, taking as a nonlimiting example a real driving emissions (RDE) standard, this new standard, coming into force in Europe, proposes calculating the emissions of pollutants, mainly nitrogen oxides NOR, carbon monoxide or CO, carbon dioxide or CO2 and particles of various species and of various diameters grouped together under the abbreviation PM under normal driving behavior in accordance with two different processes.
The first process is referred to as moving averaging windows (MAW). Such a first process evaluates the CO2 emissions over a representative evaluation window, by integration of the CO2 and uses the CO2 emissions given in grams per kilometer or g/km to standardize the real driving emissions results, by excluding so-called aberrant values that are too low or too high of more or less 50% of the CO2 emission values.
A second process uses the power measured for a classification according to power classes which have predetermined weight factors.
The two processes give NOR emissions expressed in g/km, derived from NOR emissions in grams divided by the distance in kilometers in the case of the first process, and derived from NOR emissions expressed in grams per second and divided by an average speed in kilometers per second, the two over power classes for the second process.
By referring to the first process which currently appears to be the process accepted for the tests of real driving emissions, a slightly higher or lower concentration or an exhaust mass flow rate of CO2 will not have an impact on the results of NOR in g/km, given that this modifies the distance in proportion to the variation in mass of NOR over the distance.
All the dispersion elements, such as the wind, the slope of the road, the driving style, the weight of the vehicle, the operation of air conditioning, etc., will have, in addition to the CO2 emissions and the exhaust gas flow rate, an impact on the observed NOR emissions. However, as the NOR emissions at the outlet of the exhaust line are monitored at a given point in grams per second or g/sec in accordance with the prior art relating to the monitoring of the NOR, these dispersion elements will not have an impact on the results of NOR in g/km.
However, the NOR results are a function of a variation of an average vehicle speed over the associated distance window. For example, a more aggressive driving style may lead to higher flow rates and also to higher emissions of CO2 and NOR. On the other hand, a higher average speed of the vehicle will result in lower emissions of NOR and in an overconsumption of reducing agent.
Moreover, a higher weight may lead to a higher flow rate of exhaust gas and to higher emissions of CO2 and NOR at a lower average speed which will result in a greater concentration of NOR, in other words in not meeting the requirements of the legislative standards.
Thus, in accordance with the approach according to the prior art and considering the influences mentioned above, a scenario will be defined, in the worst case, that is typically statistically representative of at least 95% of all possible cases of real driving emissions, given that 100% of the cases will never be covered. This scenario will be calibrated in terms of concentration of NOx at the end of the exhaust line in milligrams per second or mg/sec in order to meet the requirements of the real driving emissions tests, for example lower average speeds combined with higher weight and aggressive driving.
In summary, the remaining problems are, as was mentioned above, to cover at least 95% of the cases in terms of meeting NOx emission conditions, which results in an overconsumption of reducing agent.
Another problem relates to the calibrations of the engine and of the drive train which must consider all the operating conditions that will be included in the real driving emissions conditions. Not all possible cycles can be covered during the development effectively, for example the worst-case scenario of covering 95% of all possible driving cases. There is then a risk that some of the real driving emissions cycles may exceed the NOx emission limits for some of the random conditions and driving behaviors or a risk of other conditions that may vary during the real driving emissions tests. This requires the creation of a safety margin that leads to a greater consumption of reducing agent or to the use of catalysts of larger size for example.